DISCUSSION OF BACKGROUND
The present invention relates to the field of navigation aids for aircraft.
More specifically, the present invention proposes a novel method for utilizing the signals provided by an airborne radio altimeter of the FM/CW type, which are generated when an aircraft flies over a ground area which is to be analyzed.
FM/CW radio altimeters have traditional applications in the field of altimetry. They are transceivers whose carrier wave is transmitted at a frequency of, for example, a few gigahertz to a few tens of GHz and which is cyclically modulated by a frequency-modulated signal. The measurement of the frequency of the beat between the transmitted and received waves provides directly an information representative of the distance separating the transceiver from the obstacle having reflected the signal.
Traditionally, the processing of the beat signal from FM/CW radio altimeters has been performed by analog circuitry.
Now, the power density of the beat signal produced by such a system exhibits a more or less spread frequency spectrum 11, as schematically shown in FIG. 1. But, in fact, for the height measurement performed in conventional altimetry, only a small portion of the spectrum is useful and consequently used (for example, in the simple case where the attitude of the airplane is parallel to the ground, and in the absence of Doppler shift, it is the beat frequency f.sub.O corresponding to the point 10 of maximum value A.sub.m of power density A which is directly proportional to the height of the aircraft above ground).
As a consequence, in conventional altimetry with analog processing, the beat signal is very highly filtered so as to eliminate the non-utilized portions of the spectrum, in particular in the decay region 13.